Low-permittivity porous siliceous film, semiconductor devices having such films, and coating composition for forming the film

ABSTRACT

A porous silica coating having a dielectric constant of less than 2.5, a semiconductor device comprising the porous silica coating formed therein, and a coating composition for forming the porous silica coating.  
     The coating composition is composed of an aluminum-containing polysilazane and a polyacrylate or polymethacrylate ester in an organic solvent. The coating composition is coated and then fired, thereby to obtain a porous silica coating.  
     The porous silica coating can be used as an interlayer dielectric by forming on a semiconductor device.

TECHNICAL FIELD

[0001] The present invention relates to a porous silica coating with alow dielectric constant, a semiconductor device comprising the poroussilica coating, and a coating composition which becomes the poroussilica coating.

BACKGROUND ART

[0002] Polysilazane coatings are converted into silica coatings byfiring in atmospheric air. These silica coatings are used as interlayerdielectrics for semiconductors because of excellent electricalinsulating properties. Among these silica coatings, a completelyinorganic silica coating has already been employed as an excellentinterlayer dielectric for a semiconductor because it has high heatresistance and can be used in a non-etch back process. In this case, thephysical properties of the silica coating are similar to those ofsilicon dioxide (SiO₂) and its dielectric constant is within a rangefrom 3.0 to 4.7.

[0003] With the increase of the speed and integration density ofintegrated circuits, a further reduction in dielectric constant isrequired of electronic materials such as interlayer dielectrics.However, the dielectric constant of a conventional silica coating is toohigh for such a requirement. It is known to make the silica coatingporous so as to reduce the dielectric constant, however, the silicacoating generally has moisture absorption properties and the dielectricconstant increases with the elapse of time, under an ambient atmosphere.It has been proposed that a porous coating is subjected to a waterrepellent treatment thereby to add an organic group such as atrimethylsilyl group to the surface in order to prevent an increase indielectric constant with the elapse of time due to moisture absorption.However, such an additional water repellent treatment causes the problemthat the manufacturing cost increases and introduction of the organicgroup impairs an inorganic material's ability to make it possible to beused in a non-etch back process.

[0004] Thus, an object of the present invention to provide a silicacoating which makes it possible to drastically reduce the dielectricconstant (especially to less than 2.5) and to substantially maintain thereduced dielectric constant under an ambient atmosphere without beingsubjected to a water repellent treatment. Another object of the presentinvention is to provide a semiconductor device comprising the silicacoating with such a low dielectric constant as an interlayer dielectric,and a coating composition which becomes the silica coating.

DISCLOSURE OF THE INVENTION

[0005] In order to achieve the objects described above, the presentinventors have intensively studied and thus completed the presentinvention.

[0006] According to the present invention, there is provided a poroussilica coating having a dielectric constant of less than 2.5, which isobtained by firing a coating of a composition comprising analuminum-containing polysilazane and a polyacrylate or polymethacrylateester.

[0007] According to the present invention, there is also provided asemiconductor device comprising the porous silica coating as aninterlayer dielectric.

[0008] According to the present invention, there is also provided acoating composition comprising an aluminum-containing polysilazane and apolyacrylate or polymethacrylate ester in an organic solvent.

MODE FOR CARRYING OUT THE INVENTION

[0009] The porous silica coating of the present invention is obtained byfiring a coating of a composition comprising an aluminum-containingpolysilazane and a polyacrylate or polymethacrylate ester. Thealuminum-containing polysilazane is obtained by mixing a polysilazanewith an aluminum compound.

[0010] The polysilazane as a material for forming the silica coating hasin its molecular chain a silazane structure represented by the followinggeneral formula (1):

[0011] In the above formula, R¹, R² and R³ each independently representsa hydrogen atom, a hydrocarbon group, a hydrocarbon group-containingsilyl group, a hydrocarbon group-containing amino group, or ahydrocarbonoxy group. At least one of R¹ and R² represents a hydrogenatom. The hydrocarbon group may be combined with a substituent, andexamples of the substituent include halogen such as chlorine, bromineand fluorine, an alkoxy group, an alkoxycarbonyl group, and an aminogroup.

[0012] The hydrocarbon group includes an aliphatic hydrocarbon group andan aromatic hydrocarbon group, and the aliphatic hydrocarbon groupincludes a chain hydrocarbon group and a cyclic hydrocarbon group.Examples of the hydrocarbon group include an alkyl group, an alkenylgroup, a cycloalkyl group, a cycloalkenyl group, an aryl group, and anarylalkyl group. The number of carbon atoms in these hydrocarbon atomsis not limited, but is usually 20 or less, and preferably 10 or less. Inthe present invention, preferred is an alkyl group having 1 to 8 carbonatoms, and particularly 1 to 4 carbon atoms. In the hydrocarbongroup-containing silyl group, a preferable hydrocarbon group is an alkylgroup having 1 to 20 carbon atoms, and particularly 1 to 6 carbon atoms.The number of hydrocarbon atoms to be combined with Si is within a rangefrom 1 to 3. In the hydrocarbon-containing amino group andhydrocarbonoxy group, the number of carbon atoms in the hydrocarbongroup is within a range from 1 to 3.

[0013] The polysilazane having a silazane structure represented by thegeneral formula (1) in a molecular chain may be a polysilazane having achain, cyclic or crosslinked structure, or a mixture thereof. Thenumber-average molecular weight is within a range from 100 to 100,000,and preferably from 300 to 10,000. Such a polysilazane includesconventional perhydropolysilazane, organopolysilazane, and a modifiedcompound thereof.

[0014] Examples of the modified polysilazane include a platinum- orpalladium-containing polysilazane, an alcohol residue-containingpolysilazane, an HMDS (hexamethyldisilazane) residue-containingpolysilazane, an amine-containing polysilazane, and an organicacid-containing polysilazane.

[0015] For example, these modified polysilazanes are described inJapanese Unexamined Patent Publication Nos. 9-31333, 8-176512, 8-176511,and 5-345826.

[0016] The aluminum to be incorporated into the polysilazane may be analuminum compound in the form capable of being dissolved in an organicsolvent. Such a soluble aluminum compound includes an alkoxide, achelete compound, an organoaluminum, and a halide.

[0017] Examples of the alkoxide of aluminum include those represented bythe following general formula (2):

[0018] In the above formula, R⁴, R⁵ and R⁶ represent a hydrocarbongroup. The hydrocarbon group includes an aliphatic hydrocarbon group andan aromatic hydrocarbon group. The aliphatic hydrocarbon group includesa chain hydrocarbon group and a cyclic hydrocarbon group. Examples ofthe aliphatic hydrocarbon group include an alkyl group, an alkenylgroup, a cycloalkyl group, and a cycloalkenyl group. The number ofcarbon atoms is not specifically limited, but is usually 20 or less, andpreferably 8 or less. Specific examples of the aliphatic hydrocarbongroup include methyl, ethyl, propyl, butyl, pentyl, octyl, dodecyl,octadecyl, dodecenyl, cyclohexyl, and cyclohexenyl. The aromatichydrocarbon group includes an aryl group and an arylalkyl group.Specific examples of the aromatic hydrocarbon group include phenyl,tolyl, xylyl, naphthyl, benzyl, phenethyl, and naphthylmethyl.

[0019] Examples of the chelete compound of aluminum include aluminumacetylacetonate and aluminum ethylacetonate.

[0020] Examples of the organoaluminum include those represented by thefollowing general formula (3):

[0021] In the above formula, R⁴, R⁵ and R⁶ represent a hydrocarbongroup. The hydrocarbon group includes those described in connection withthe general formula (2).

[0022] Examples of the halide of aluminum include those represented bythe following general formula (4):

AlX₃  (4)

[0023] wherein X represents a halogen. The halogen includes chlorine,bromine, iodine, and fluorine.

[0024] The organic solvent-soluble aluminum compounds can be used aloneor in combination.

[0025] The amount of the aluminum compound to be added to thepolysilazane varies depending on the kind, but is within a range from0.001 to 10% by weight as aluminum, preferably from 0.01 to 10% byweight, and more preferably from 0.1 to 1% by weight, on the basis ofthe polysilazane. When the amount of the aluminum compound is largerthan the range described above, the density and homogeneity of theresulting silica coating are lowered. Therefore, it is not preferred. Onthe other hand, when the amount is smaller than the range, the effect ofthe added aluminum compound becomes insufficient.

[0026] To obtain the aluminum-containing polysilazane, a polysilazaneand an aluminum compound are mixed with stirring in an organic solvent.In this case, they are mixed with stirring under the conditions of atemperature within a range from 0 to 200° C., preferably from 0 to 100°C., and a pressure within a range from normal pressure to 10 kg/cm²G,preferably normal pressure. The concentration of the polysilazane in theorganic solvent is within a range from 0.1 to 80% by weight, andpreferably from 5 to 50% by weight.

[0027] As the organic solvent in which the polysilazane and aluminumcompound are dissolved, an inert organic solvent free from activehydrogen is used. Examples of the organic solvent include an aromatichydrocarbon solvent such as benzene, toluene, xylene, ethylbenzene,diethylbenzene, trimethylbenzene, or triethylbenzene; an alicyclichydrocarbon solvent such as cyclohexane, cyclohexene,decahydronaphthalene, ethylcyclohexane, methylcyclohexane, p-menthine,or dipentene (limonene); an ether solvent such as dipropyl ether ordibutyl ether; and a ketone solvent such as methyl isobutyl ketone.

[0028] An aluminum-containing polysilazane, in which an aluminumcompound is mixed or added, is formed by mixing the polysilazane and thealuminum compound with stirring in the organic solvent. Usually, theresulting aluminum-containing polysilazane does not have analuminopolysilazane structure wherein aluminum and silicon are firmlycombined.

[0029] The coating composition of the present invention is obtained byadding a polyacrylate or polymethacrylate ester to an organic solventsolution containing the aluminum-containing polysilazane thus obtained.

[0030] The polyacrylate or polymethacrylate ester, which is useful inthe present invention, is a homopolymer or copolymer of a polyacrylateor polymethacrylate ester, and specific examples thereof includepolymethyl acrylate, polyethyl acrylate, polybutyl acrylate, polymethylmethacrylate, polyethyl methacrylate, polybutyl methacrylate,polyisobutyl methacrylate, and block copolymers and other copolymersthereof.

[0031] As the polyacrylate or polymethacrylate ester in the presentinvention, those having a number-average molecular weight within a rangefrom 1,000 to 800,000 are used. When the number-average molecular weightis smaller than 1,000, a porous coating is not formed because thepolyacrylate or polymethacrylate ester is sublimated at low temperature.When the number-average molecular weight exceeds 800,000, the pore sizeincreases to cause voids, thus reducing the coating strength. Therefore,both cases are not preferred. The number-average molecular weight ofthe-polyacrylate or polymethacrylate ester in the present invention ispreferably within a range from 10,000 to 600,00, and particularlypreferred results are obtained when the number-average molecular weightis within a range from 50,000 to 300,000.

[0032] The amount of the polyacrylate or polymethacrylate ester in thepresent invention is controlled within a range from 5 to 150% by weightbased on the polysilazane used. When the amount of the polyacrylate orpolymethacrylate ester is smaller than 5% by weight, the coating isinsufficiently made porous. On the other hand, when the amount is largerthan 150% by weight, defects such as voids and cracks occur, thereby toreduce the coating strength. Therefore, it is not preferred. The amountof the polyacrylate or polymethacrylate ester in the present inventionis preferably within a range from 10 to 120% by weight, and particularlypreferred results are obtained when the amount is within a range from 20to 100% by weight.

[0033] The polyacrylate or polymethacrylate ester is generally added toan aluminum-containing polysilazane solution in the form of a solutionprepared by dissolving the polyester in an organic solvent. In thiscase, the same organic solvent as that used in preparation of thealuminum-containing polysilazane solution may be used as the organicsolvent. As the organic solvent in which the polyacrylate orpolymethacrylate ester is dissolved, an inert organic solvent free fromactive hydrogen described above is used. When using the polyacrylate orpolymethacrylate ester after dissolving in the organic solvent, theconcentration of the polyacrylate or polymethacrylate ester can becontrolled within a range from 5 to 80% by weight, and preferably from10 to 40% by weight. A homogeneous solution can be obtained byphysically stirring after the addition of the polyacrylate orpolymethacrylate ester.

[0034] The polyacrylate or polymethacrylate ester itself can also beadded and dissolved in the aluminum-containing polysilazane solution.The coating composition of the present invention can be prepared bycombining the polysilazane with the polyacrylate or polymethacrylateester and mixing the aluminum compound therewith, or combining thepolyacrylate or polymethacrylate ester with the aluminum compound andmixing the polysilazane therewith.

[0035] The resulting organic solvent solution containing thealuminum-containing polysilazane and the polyacrylate orpolymethacrylate ester can be coated on the surface of a substrate byusing it as a coating composition with or without controlling theconcentration of the polysilazane.

[0036] Examples of the method of coating the coating compositioncontaining the aluminum-containing polysilazane and the polyacrylate orpolymethacrylate ester to the surface of the substrate includeconventionally known methods, for example, spin coating method, dippingmethod, spraying method, and transferring method.

[0037] The aluminum-containing polysilazane coating formed on thesurface of the substrate is fired in various atmospheres. The atmosphereincludes, for example, an atmosphere which scarcely contains watervapor, such as dry air, dry nitrogen, or dry helium, or an atmospherecontaining water vapor, such as atmospheric air, moistened atmosphericair, or moistened nitrogen. The firing temperature is within a rangefrom 50 to 600° C., and preferably from 300 to 500° C., and the firingtime is within a range from one minute to one hour.

[0038] According to the present invention, a silica coating having a lowdielectric constant and a good coating quality is advantageouslyprepared by forming a polysilazane coating on the surface of asubstrate, preliminary heating the coating in a water vapor-containingatmosphere and firing the coating with heating in a dry atmosphere. Inthis case, in the water vapor-containing atmosphere, the water vaporcontent is 0.1 volume % or more, and preferably 1 volume % or more. Theupper limit value is the dew point. Examples of such an atmosphereinclude atmospheric air, moistened atmospheric air, and moistenednitrogen gas. In the dry atmosphere, the water vapor content is 0.5volume % or less, and preferably 0.05 volume % or less. Examples of thedry atmosphere include dry air, nitrogen gas, argon gas, and helium gas.The preliminary heating temperature is within a range from 50 to 400°C., and preferably from 100 to 350° C. The firing temperature is withina range from 100 to 500° C., and preferably from 300 to 500° C.

[0039] In the firing of the aluminum-containing polysilazane coating,Si—H, Si—R (R: hydrocarbon group) and Si—N bonds in the polysilazane areoxidized and converted into Si—O bonds to form a silica coating. In thiscase, a Si—OH bond is not substantially formed. Generally, in the firingof the polysilazane coating with heating, Si—H, Si—R and Si—N bonds areoxidized nearly simultaneously, although it varies depending on theconditions of firing. This fact is confirmed from the fact thatabsorptions based on Si—H, Si—R and Si—N disappear nearly simultaneouslywhen the IR spectrum of the resulting silica coating is measured.According to the present inventors study, it was confirmed that, in caseof the firing of the aluminum-containing polysilazane coating used inthe present invention with heating, the oxidation of the Si—N bond,namely, the reaction of substituting N with O preferentially proceeds ascompared with the oxidation of the Si—H and Si—R bonds by a catalyticaction of aluminum.

[0040] Therefore, the present invention allows the Si—O bond formed byselectively oxidizing the Si—N bond, and the unoxidized Si—H and Si—Rbonds, to exist in the formed silica coating, thereby making it possibleto obtain a silica coating with a low density. Generally, the dielectricconstant of the silica coating is reduced with the reduction of thecoating density, while adsorption of water as a high dielectricsubstance occurs when the coating density is reduced. Therefore, therearises a problem that the dielectric constant increases when the silicacoating is left to stand in an atmospheric air. In the case of thesilica coating containing Si—H and Si—R bonds of the present invention,adsorption of water can be prevented regardless of low density becausethese bonds have water repellency. Therefore, the silica coating of thepresent invention has a large merit that the dielectric constant of thecoating scarcely increases even if the silica coating is left to standin an atmospheric air containing water vapor. The silica coating of thepresent invention also has a merit that it is less likely to causecracking because the internal stress of the coating is small due to lowdensity.

[0041] In the firing of the coating, micropores having a diameter of 0.5to 30 nm are formed in the silica coating by sublimation of thepolyacrylate of polymethacrylate ester in the coating. The existence ofthe micropores further reduces the density of the silica coating, andthus the dielectric constant of the silica coating is further reduced.This is because the compatibility between the aluminum-containingpolysilazane and the polyacrylate of polymethacrylate ester is verygood. The use of the polyacrylate of polymethacrylate ester prevents theSi—OH bond from forming in the polysilazane during the firing of thecoating. Therefore, the silica coating maintains the water repellencyand the dielectric constant reduced due to the micropores scarcelyincreases even when left to stand in an atmospheric air containing watervapor. As described above, according to the present invention, it ismade possible to obtain a porous silica coating capable of stablymaintaining a very low dielectric constant of less than 2.5, preferably2.0 or less, occasionally about 1.6, in cooperation with the reductionin density and impartation of water repellency due to the bond component(SiH, SiR) of the silica coating as well as reduction in density of thewhole coating due to micropores. Therefore, since a water repellenttreatment required to prevent moisture absorption in a conventionalporous silica coating is not required, it becomes advantageous in viewof the manufacturing cost and an inorganic material's merit is notimpaired by introduction of an organic group.

[0042] Referring to other properties of the silica coating of thepresent invention, the density is within a range from 0.5 to 1.4 g/cm³,and preferably from 0.7 to 1.1 g/cm³, and the cracking limitation incoating thickness is 1.0 μm or more, and preferably 10 μm or more and,furthermore, the internal stress is 2.0×10⁴ N/cm² or less, andpreferably 1.0×10⁴ N/cm² or less. The content of Si, which exists in theform of a Si—H or Si—R bond (R: hydrocarbon group), in the silicacoating is within a range from 10 to 100 atomic %, and preferably from25 to 75 atomic %, based on the number of Si atoms contained in thesilica porous coating. The content of Si, which exists in the form of aSi—N bond, is 5 atomic % or less.

[0043] The thickness of the silica coating obtained after firing variesdepending on the purposes of the substrate surface, but is usuallywithin a range from 0.01 to 5 μm, and preferably from 0.1 to 2 μm. Whenusing as an interlayer dielectric, the thickness is within a range from0.1 to 2 μm.

[0044] In the present invention, when using perhydropolysilazane havingno hydrocarbon group as the polysilazane, it is made possible to obtainan inorganic coating with a low dielectric constant, which is composedonly of elements of Si, O and H and has a Si—H bond but substantiallyhas neither an N—H bond nor a Si—OH bond. Since this coating is superiorin resistance to plasma, a so-called etch-back process of removing acoating layer on the metal wiring in a process of manufacturing asemiconductor can be omitted by applying this coating in the manufactureof the semiconductor. Therefore, the process of manufacturing thesemiconductor can be markedly simplified.

[0045] As described above, the porous silica coating of the presentinvention has a low density and has a merit that a cracking limitationin coating thickness, namely, a maximum coating thickness where acoating can be formed without causing cracking of the coating is 5 μm ormore. In case of a conventional silica coating, the cracking limitationin coating thickness is within a range from about 0.5 to 1.5 μm.Therefore, the silica coating of the present invention exhibits a largetechnical effect as compared with a conventional silica coating.

[0046] The method of forming the silica coating of the present inventioncan be carried out very easily because the aluminum-containingpolysilazane, as a precursor thereof, can be converted into a silicacoating at a firing temperature lower than 450° C. even in a dryatmospheric air by a catalytic action of aluminum.

[0047] Therefore, the present invention is advantageously applied as amethod of forming an insulating coating to an aluminum wiring whoseheat-resistant upper limit temperature is 450° C. According to thepresent invention, since the content of the Si—N bond can besubstantially reduced to 0% by a catalytic action of aluminum, thecoating has very high stability and is not deteriorated even when leftto stand in atmospheric air.

[0048] The silica coating of the present invention can be advantageouslyused as an interlayer dielectric in a semiconductor device. In thiscase, the silica coating is formed on the plane including a metalwiring, or a metal wiring with a ceramic coating. The semiconductordevice including the silica coating of the present invention is not onlysuperior in insulating properties, but also in electric characteristicsbecause of small dielectric constant.

[0049] A silica coating can be formed on the solid surface of variousmaterials such as metal, ceramics or lumber by using the coatingcomposition of the present invention. According to the presentinvention, there are provided a metal substrate (silicon, stainlesssteel (SUS), tungsten, iron, copper, zinc, brass, or aluminum) with asilica coating formed thereon, and a ceramics substrate (metal oxidesuch as silica, alumina, magnesium oxide, titanium oxide, zinc oxide andtantalum oxide, metal nitride such as silicon nitride, boron nitride andtitanium nitride, or silicon carbide) with a silica coating formedthereon.

[0050] The following Examples further illustrate the present inventionin detail.

[0051] The method of evaluating physical properties of the silicacoating is as follows.

[0052] (Dielectric Constant)

[0053] A Pyrex glass plate (thickness: 1 mm, size: 50 mm×50 mm)manufactured by Dow Corning Inc. was sufficiently washed, in order, witha neutral detergent, an aqueous diluted NaOH solution and an aqueousdiluted H₂SO₄ solution, and then dried. An Al coating (0.2 A) was formedon the whole surface of the glass plate by a vacuum deposition method.After coating the glass plate with a polysilazane solution by a spincoating method, the resulting polysilazane coating (about 3 mm×3 mm insize) was removed by rubbing, with a rod applicator, four corners of theglass plate to form portions for taking electric signals. Subsequently,the polysilazane coating was converted into a silica coating inaccordance with the method of the Examples or Comparative Examples. Theresulting silica coating was covered with a mask of SUS and an Alcoating was formed by a vacuum deposition method (18 patterns in theform of square of 2 mm×2 mm, 2 μm in thickness). A capacitance wasmeasured by an impedance analyzer 4192 ALF manufactured by YHP Inc. (100kHz). The thickness of the coating was measured by a profilometer(Dektak IIA manufactured by Sloan Inc.). The dielectric constant wascalculated by the following equation.

Dielectric constant=(Capacitance [pF])×(Coating thickness [μm])/35.4

[0054] The dielectric constant was determined by calculating an averageof 18 measured values.

[0055] (Coating Density)

[0056] The weight of a silicon wafer, 4 inch in diameter and 0.5 mm inthickness, was measured by an electric balance. After coating thesilicon wafer with a polysilazane solution by a spin coating method, theresulting polysilazane coating was converted into a silica coating inaccordance with the method of the Examples or Comparative Examples andthe weight of the coated silicon wafer was measured again by theelectric balance. A difference in weight was taken as the weight of thecoating. In the same manner as in case of the evaluation of thedielectric constant, the thickness of the coating was measured by aprofilometer (Dektak IIA manufactured by Sloan Inc.). The coatingdensity was calculated by the following equation.

Coating density [g/cm ³]=(Coating weight [g])×(Coating thickness[μm])/0.008.

[0057] (Internal Stress)

[0058] Data of warp of a silicon wafer, 4 inch in diameter and 0.5 mm inthickness, were input in a laser thin film internal stress measurementsystem Model FLX-2320 manufactured by Tencor Corporation. After coatingthe silicon wafer with a polysilazane solution by a spin coating method,the resulting polysilazane coating was converted into a silica coatingin accordance with the method of the Examples or Comparative Examplesand cooled to room temperature (23° C.). Then, the internal stress wasmeasured by the laser thin film internal stress measurement system ModelFLX-2320 manufactured by Tencor Corporation. In the same manner as incase of the evaluation of the dielectric constant, the thickness of thecoating was measured by a profilometer (Dektak IIA manufactured by SloanInc.).

[0059] (Cracking Limitation in Coating Thickness)

[0060] After coating a silicon wafer, 4 inch in diameter and 0.5 mm inthickness, with a polysilazane solution by a spin coating method, theresulting polysilazane coating was converted into a silica coating inaccordance with the method of the Examples or Comparative Examples.Samples having coating different thicknesses within a range from about0.5 to 3μ were made by controlling the polysilazane concentration of thepolysilazane solution or the rotational speed of a spin coater. Thefired thin coating was observed by a microscope (magnification: ×120)and it was examined whether or not cracking occurred. A maximum coatingthickness where no cracking occurs was taken as a cracking limitation incoating thickness.

REFERENCE EXAMPLE Synthesis of Perhydropolysilazane

[0061] A four-necked flask having an internal volume of 2 L was equippedwith a gas bubbling tube, a mechanical scaler and a Dewar condenser.After replacing the atmosphere of a reaction vessel by dry nitrogen,1500 ml of dry pyridine was charged in the four-necked flask and thenice-cooled. 100 g of dichlorosilane was added to produce an adduct as awhite solid (SiH₂Cl₂ 2C₅H₅N). The reaction mixture was ice-cooled and 70g of ammonia was bubbled into the reaction mixture while stirring.Subsequently, dry nitrogen was bubbled into the aqueous layer for 30minutes to remove excess ammonia.

[0062] The resulting product was removed by filtering through a Buchnerfunnel under reduced pressure in a dry nitrogen atmosphere to obtain1200 ml of a filtrate. Pyridine was distilled off by an evaporator toobtain 40 g of perhydropolysilazane.

[0063] The number-average molecular weight of the resultingperhydropolysilazane was measured by GPC (developing solution: CDCl₃).As a result, it was 800 calibrated with polystyrene standards. An IR(infrared absorption) spectrum showed absorptions based on N—H at a wavenumber of approximately 3350 and 1200 cm⁻¹, an absorption based on Si—Hat 2170 cm⁻¹, and an absorption based on Si—N—Si at 1020 to 820 cm

Comparative Example 1

[0064] 20 g of perhydropolysilazane synthesized in Reference Example 1was dissolved in 80 g of xylene to prepare a polysilazane solution.Subsequently, the polysilazane solution was filtered through a PTFEsyringe filter having a filtration accuracy of 0.2 μm manufactured byAdvantech Co., Ltd. The filtered polysilazane solution was coated on asilicon wafer of 4 inch in diameter and 0.5 mm in thickness using a spincoater (1500 rpm, 20 seconds), and then dried at room temperature (10minutes). The silicon plate coated with polysilazane was heated on a hotplate at 100° C., then at 200° C. in an atmospheric air (25° C.,relative humidity: 40%) each for 3 minutes. The heated silicon plate wasfired in a dry nitrogen atmosphere at 400° C. for one hour. Absorptionsbased on Si—O at a wave number of 1020 and 450 cm⁻¹ was observed.Absorptions of unconverted polysilazane, namely, absorptions based onN—H at a wave number of 3380 and 1200 cm⁻¹ and absorptions based on Si—Hat a wave number of 2210 and 860 cm⁻¹ were observed. The resultingcoating was evaluated. As a result, the coating had a dielectricconstant of 4.2, a density of 1.8 g/cm³, an internal stress of 1.2×10⁴N/cm², and a cracking limitation in coating thickness of 2.2 μm. Theresulting coating was left to stand in an atmospheric air under theconditions of a temperature of 23° C. and a relative humidity of 50% fora week and the dielectric constant was measured again. As a result, itwas 4.8.

Comparative Example 2

[0065] 25 g of perhydropolysilazane synthesized in Reference Example 1was dissolved in 55 g of xylene to prepare a polysilazane solution.Subsequently, 0.1 g of tri(isopropoxy)aluminum was mixed with 20 g ofxylene and sufficiently dissolved. The resulting solution was mixed withthe polysilazane solution. The mixed solution was filtered through aPTFE syringe filter having a filtration accuracy of 0.2 μm manufacturedby Advantech Co., Ltd. The filtered solution was coated on a siliconwafer, 4 inch in diameter and 0.5 mm in thickness, using a spin coater(1500 rpm, 20 seconds), and then dried at room temperature (10 minutes).The silicon plate coated with polysilazane was heated on a hot plate at100° C., then at 200° C. in an atmospheric air (25° C., relativehumidity: 40%) for each 3 minutes. The heated silicon plate was fired ina dry nitrogen atmosphere at 400° C. for one hour. Absorptions based onSi—O at a wave number of 1070 and 450 cm⁻¹ and absorptions based on Si—Hat a wave number of 2250 and 880 cm⁻¹ were mainly observed, whileabsorptions based on N—H at a wave number of 3350 and 1200 cm⁻¹ nearlydisappeared. The resulting coating was evaluated. As a result, thecoating had a dielectric constant of 3.0, a density of 2.9 g/cm³, aninternal stress of 1.2×10⁴ N/cm², and a cracking limitation in coatingthickness of 1.4 μm. The resulting coating was left to stand in anatmospheric air under the conditions of a temperature of 23° C. and arelative humidity of 50% for a week and the dielectric constant wasmeasured again. As a result, it was 3.2.

Comparative Example 3

[0066] 25 g of perhydropolysilazane synthesized in Reference Example 1was dissolved in 55 g of xylene to prepare a polysilazane solution.Subsequently, 0.1 g of tri(ethylacetoacetate)aluminum was mixed with 20g of xylene and sufficiently dissolved. The resulting solution was mixedwith the polysilazane solution. The mixed solution was filtered througha PTFE syringe filter having a filtration accuracy of 0.2 μmmanufactured by Advantech Co., Ltd. The filtered solution was coated ona silicon wafer, 4 inch in diameter and 0.5 mm in thickness, using aspin coater (1500 rpm, 20 seconds), and then dried at room temperature(10 minutes). The silicon plate coated with polysilazane was heated on ahot plate at 150° C., then at 220° C. in an atmospheric air (25° C.,relative humidity: 40%) each for 3 minutes. The heated silicon plate wasfired in a dry nitrogen atmosphere at 400° C. for one hour. Absorptionsbased on Si—O at a wave number of 1065 and 460 cm⁻¹ and absorptionsbased on Si—H at a wave number of 2250 and 830 cm⁻¹ were mainlyobserved, while absorptions based on N—H at a wave number of 3350 and1200 cm⁻¹ nearly disappeared. The resulting coating was evaluated. As aresult, the coating had a dielectric constant of 2.3, a density of 1.7g/cm³, an internal stress of 1.2×10⁴ N/cm², and a cracking limitation incoating thickness of 1.3 μm. The resulting coating was left to stand inan atmospheric air under the conditions of a temperature of 23° C. and arelative humidity of 50% for a week and the dielectric constant wasmeasured again. As a result, it was 2.5.

Example 1

[0067] 30 g of perhydropolysilazane synthesized in Reference Example 1was dissolved in 120 g of xylene to prepare a polysilazane solution.Subsequently, 3 g of tri(acetylacetonato)aluminum was mixed with 97 g ofxylene and sufficiently dissolved. 1 g of the solution from theresulting solution was mixed with the polysilazane solution. A solutionprepared by sufficiently dissolving 15 g of polymethyl methacrylatehaving a molecular weight of about 95,000 in 60 g of xylene was mixedwith the polysilazane solution, followed by stirring sufficiently. Themixed solution was filtered through a PTFE syringe filter having afiltration accuracy of 0.2 μm manufactured by Advantech Co., Ltd. Thefiltered solution was coated on a silicon wafer of 4 inch in diameterand 0.5 mm in thickness using a spin coater (2000 rpm, 20 seconds), andthen dried at room temperature (5 minutes). The silicon plate coatedwith polysilazane was heated on a hot plate at 150° C., then at 220° C.in an atmospheric air (25° C., relative humidity: 40%) for each 3minutes. The heated silicon plate was fired in a dry nitrogen atmosphereat 400° C. for 30 minutes. Absorptions based on Si—O at a wave number of1060 and 450 cm⁻¹ and absorptions based on Si—H at a wave number of 2250and 880 cm⁻¹ were mainly observed, while absorptions based on N—H at awave number of 3350 and 1200 cm⁻¹ and absorption based on polymethylmethacrylate disappeared. The resulting coating was evaluated. As aresult, the coating had a dielectric constant of 1.9, a density of 0.8g/cm³, an internal stress of 2.6×10³ N/cm², and a cracking limitation incoating thickness of 5 μm or more. The resulting coating was left tostand in an atmospheric air under the conditions of a temperature of 23°C. and a relative humidity of 50% for a week and the dielectric constantwas measured again. As a result, it was 2.0.

Example 2

[0068] 30 g of perhydropolysilazane synthesized in Reference Example 1was dissolved in 120 g of dibutyl ether to prepare a polysilazanesolution. Subsequently, 3 g of aluminumtris(ethyl acetoacetate) wasmixed with 97 g of dibutyl ether and sufficiently dissolved. 2 g of thesolution from the resulting solution was mixed with the polysilazanesolution. A solution prepared by sufficiently dissolving 15 g ofpoly(isobutyl) methacrylate having a molecular weight of about 180,000in 60 g of dibutyl ether was mixed with the polysilazane solution,followed by stirring sufficiently. The mixed solution was filteredthrough a PTFE syringe filter having a filtration accuracy of 0.2 μmmanufactured by Advantech Co., Ltd. The filtered solution was coated ona silicon wafer, 4 inch in diameter and 0.5 mm in thickness, using aspin coater (2000 rpm, 20 seconds), and then dried at room temperature(5 minutes). The silicon plate coated with polysilazane was heated on ahot plate at 150° C., then at 220° C. in an atmospheric air (25° C.,relative humidity: 40%) for each 3 minutes. The heated silicon plate wasfired in a dry nitrogen atmosphere at 400° C. for 30 minutes.Absorptions based on Si—O at a wave number of 1070 and 455 cm⁻¹ andabsorptions based on Si—H at a wave number of 2300 and 850 cm⁻¹ weremainly observed, while absorptions based on N—H at a wave number of 3350and 1200 cm⁻¹ and absorption based on poly(isobutyl) methacrylatedisappeared. The resulting coating was evaluated. As a result, thecoating had a dielectric constant of 2.0, a density of 1.0 g/cm³, aninternal stress of 3.1×10³ N/cm², and a cracking limitation in coatingthickness of 5 μm or more. The resulting coating was left to stand in anatmospheric air under the conditions of a temperature of 23° C. and arelative humidity of 50% for a week and the dielectric constant wasmeasured again. As a result, it was 2.1.

Example 3

[0069] 20 g of perhydropolysilazane synthesized in Reference Example 1was dissolved in 80 g of xylene to prepare a polysilazane solution.Subsequently, 2 g of tri(ethylacetoacetate)aluminum was mixed with 98 gof xylene and sufficiently dissolved. 1 g of the solution from theresulting solution was mixed with the polysilazane solution. A solutionprepared by sufficiently dissolving 20 g of BR80 manufactured byMitsubishi Rayon Co., Ltd. in 80 g of xylene was mixed with thepolysilazane solution, followed by stirring sufficiently. The mixedsolution was filtered through a PTFE syringe filter having a filtrationaccuracy of 0.2 μm manufactured by Advantech Co., Ltd. The filteredsolution was coated on a silicon wafer, 4 inch in diameter and 0.5 mm inthickness, using a spin coater (2000 rpm, 20 seconds), and then dried atroom temperature (5 minutes). The silicon plate coated with polysilazanewas heated on a hot plate at 150° C., then at 220° C. in an atmosphericair (25° C., relative humidity: 40%) for each 3 minutes. The heatedsilicon plate was fired in a dry nitrogen atmosphere at 400° C. for 30minutes. Absorptions based on Si—O at a wave number of 1075 and 470 cm⁻¹and absorptions based on Si—H at a wave number of 2250 and 840 cm⁻¹ weremainly observed, while absorptions based on N—H at a wave number of 3350and 1200 cm⁻¹ and absorption based on BR80 disappeared. The resultingcoating was evaluated. As a result, the coating had a dielectricconstant of 1.6, a density of 0.8 g/cm³, an internal stress of 1.8×10³N/cm², and a cracking limitation in coating thickness of 5 μm or more.The resulting coating was left to stand in an atmospheric air under theconditions of a temperature of 23° C. and a relative humidity of 50% fora week and the dielectric constant was measured again. As a result, itwas 1.6.

Example 4

[0070] 20 g of perhydropolysilazane synthesized in Reference Example 1was dissolved in 80 g of dibutyl ether to prepare a polysilazanesolution. Subsequently, 2 g of aluminumtris(ethylacetoacetate) was mixedwith 98 g of dibutyl ether and sufficiently dissolved. 2 g of thesolution from the resulting solution was mixed with the polysilazanesolution. A solution prepared by sufficiently dissolving 10 g of BR1122manufactured by Mitsubishi Rayon Co., Ltd. in 40 g of dibutyl ether wasmixed with the polysilazane solution, followed by stirring sufficiently.The mixed solution was filtered through a PTFE syringe filter having afiltration accuracy of 0.2 μm manufactured by Advantech Co., Ltd. Thefiltered solution was coated on a silicon wafer, 4 inch in diameter and0.5 mm in thickness, using a spin coater (2000 rpm, 20 seconds), andthen dried at room temperature (5 minutes). The silicon plate coatedwith polysilazane was heated on a hot plate at 150° C., 220° C., then at300° C. in an atmospheric air (25° C., relative humidity: 40%) for each3 minutes. The heated silicon plate was fired in a dry nitrogenatmosphere at 400° C. for 30 minutes. Absorptions based on Si—O at awave number of 1068 and 435 cm⁻¹ and absorptions based on Si—H at a wavenumber of 2300 and 830 cm⁻¹ were mainly observed, while absorptionsbased on N—H at a wave number of 3350 and 1200 cm⁻¹ and absorption basedon BR1122 disappeared. The resulting coating was evaluated. As a result,the coating had a dielectric constant of 1.9, a density of 0.9 g/cm³, aninternal stress of 2.8×10³ N/cm², and a cracking limitation in coatingthickness of 5 μm or more. The resulting coating was left to stand in anatmospheric air under the conditions of a temperature of 23° C. and arelative humidity of 50% for a week and the dielectric constant wasmeasured again. As a result, it was 2.0.

Example 5

[0071] 40 g of perhydropolysilazane synthesized in Reference Example 1was dissolved in 160 g of xylene to prepare a polysilazane solution.Subsequently, 2 g of tri(isopropoxy)aluminum was mixed with 98 g ofxylene and sufficiently dissolved. 6 g of the solution from theresulting solution was mixed with the polysilazane solution. A solutionprepared by sufficiently dissolving 10 g of BR80 manufactured byMitsubishi Rayon Co., Ltd. in 40 g of xylene was mixed with thepolysilazane solution, followed by stirring sufficiently. The mixedsolution was filtered through a PTFE syringe filter having a filtrationaccuracy of 0.2 μm manufactured by Advantech Co., Ltd. The filteredsolution was coated on a silicon wafer, 4 inch in diameter and 0.5 mm inthickness, using a spin coater (2000 rpm, 20 seconds), and then dried atroom temperature (5 minutes). The silicon plate coated with polysilazanewas heated on a hot plate at 150° C., then at 220° C. in an atmosphericair (25° C., relative humidity: 40%) for each 3 minutes. The heatedsilicon plate was fired in a dry nitrogen atmosphere at 400° C. for 30minutes. Absorptions based on Si—O at a wave number of 1070 and 450 cm⁻¹and absorptions based on Si—H at a wave number of 2250 and 880 cm⁻¹ weremainly observed, while absorptions based on N—H at a wave number of 3350and 1200 cm⁻¹ and absorption based on BR80 disappeared. The resultingcoating was evaluated. As a result, the coating had a dielectricconstant of 1.8, a density of 1.0 g/cm³, an internal stress of 2.7×10³N/cm², and a cracking limitation in coating thickness of 5 μm or more.The resulting coating was left to stand in an atmospheric air under theconditions of a temperature of 23° C. and a relative humidity of 50% fora week and the dielectric constant was measured again. As a result, itwas 2.0.

INDUSTRIAL APPLICABILITY

[0072] The porous silica coating of the present invention has a lowdensity and a low dielectric constant of less than 2.5, in addition tothe chemical resistance, gas/ion barrier properties, wear resistance,heat resistance and flattening properties which are inherent in a silicacoating derived from a polysilazane. This porous silica coating scarcelyadsorbs water vapor even when left to stand in an atmospheric airbecause it contains a hydrophobic Si—H bond and, therefore, thedielectric constant is less likely to increase. Moreover, the poroussilica coating of the present invention has a feature that it has smallcoating stress and high coating thickness limitation. Accordingly, theporous silica coating of the present invention is suited for use as aninterlayer dielectric in semiconductors.

[0073] The porous silica coating of the present invention is preferablyused as the interlayer dielectric in semiconductors, and is alsoadvantageously used as an insulating coating in the electrical andelectronic fields, such as under coating (insulating flattened coating)of liquid crystal glass and gas barrier coating of film liquid crystal.

[0074] The method of forming a porous silica coating of the presentinvention can be applied as, for example, a hard coating onto thesurface of a solid such as metal, glass, plastic or lumber,heat-resistant coating, acid-resistant coating, stainproof coating, andwater repellent coating. It can also be applied as gas barrier coatingonto a plastic film, UV cut coating onto a glass, plastic or lumber, anda coloring coating.

[0075] The coating composition can be applied as UV cut coating,coloring coating and antibacterial coating because various functionalfillers can be added.

[0076] The porous silica coating of the present invention isadvantageous in view of the manufacturing cost because it is notrequired to be subjected to a water repellent treatment for preventionof moisture absorption and, moreover, an inorganic material's merit isnot impaired by the introduction of the organic group.

1. A porous silica coating having a dielectric constant of less than2.5, which is obtained by firing a coating of a composition comprisingan aluminum-containing polysilazane and a polyacrylate orpolymethacrylate ester.
 2. The porous silica coating according to claim1, which maintains a dielectric constant of less than 2.5 even afterbeing left to stand in an atmospheric air at a temperature of 23° C. anda relative humidity of 50% for a week.
 3. The porous silica coatingaccording to claim 1 or 2, wherein the dielectric constant is 2.1 orless.
 4. The porous silica coating according to claim 1, which has apore diameter within a range from 0.5 to 30 nm.
 5. The porous silicacoating according to claim 1, wherein the polysilazane in thealuminum-containing polysilazane has a silazane structure represented bythe following general formula:

wherein R¹, R² and R³ each independently represents a hydrogen atom, ahydrocarbon group, a hydrocarbon group-containing silyl group, ahydrocarbon group-containing amino group, or a hydrocarbonoxy group,provided that at least one of R¹ and R² represents a hydrogen atom. 6.The porous silica coating according to claim 5, wherein the content ofSi, which exists in the form of a Si—R¹ or Si—R² bond, is within a rangefrom 10 to 100 atomic % based on the number of Si atoms contained in thesilica porous coating.
 7. The porous silica coating according to claim6, wherein all of R¹, R² and R³ are hydrogen atoms.
 8. The porous silicacoating according to claim 6 or 7, which is substantially free from aSi—N bond.
 9. A semiconductor device comprising the porous silicacoating of claim 1 as an interlayer dielectric.
 10. (deleted)